lib/daemon.c

   1 /*
   2  * Copyright (c) 2014 Nicira, Inc.
   3  *
   4  * Licensed under the Apache License, Version 2.0 (the "License");
   5  * you may not use this file except in compliance with the License.
   6  * You may obtain a copy of the License at:
   7  *
   8  *     http://www.apache.org/licenses/LICENSE-2.0
   9  *
  10  * Unless required by applicable law or agreed to in writing, software
  11  * distributed under the License is distributed on an "AS IS" BASIS,
  12  * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
  13  * See the License for the specific language governing permissions and
  14  * limitations under the License.
  15  */
  16 #include <config.h>
  17 #include "daemon.h"
  18 #include <errno.h>
  19 #include <fcntl.h>
  20 #include <unistd.h>
  21 #include "vlog.h"
  22
  23 VLOG_DEFINE_THIS_MODULE(daemon);
  24
  25 /* For each of the standard file descriptors, whether to replace it by
  26  * /dev/null (if false) or keep it for the daemon to use (if true). */
  27 static bool save_fds[3];
  28
  29 /* A daemon doesn't normally have any use for the file descriptors for stdin,
  30  * stdout, and stderr after it detaches.  To keep these file descriptors from
  31  * e.g. holding an SSH session open, by default detaching replaces each of
  32  * these file descriptors by /dev/null.  But a few daemons expect the user to
  33  * redirect stdout or stderr to a file, in which case it is desirable to keep
  34  * these file descriptors.  This function, therefore, disables replacing 'fd'
  35  * by /dev/null when the daemon detaches. */
  36 void
  37 daemon_save_fd(int fd)
  38 {
  39     ovs_assert(fd == STDIN_FILENO ||
  40                fd == STDOUT_FILENO ||
  41                fd == STDERR_FILENO);
  42     save_fds[fd] = true;
  43 }
  44
  45 /* Returns a readable and writable fd for /dev/null, if successful, otherwise
  46  * a negative errno value.  The caller must not close the returned fd (because
  47  * the same fd will be handed out to subsequent callers). */
  48 static int
  49 get_null_fd(void)
  50 {
  51     static int null_fd;
  52 #ifndef _WIN32
  53     char *device = "/dev/null";
  54 #else
  55     char *device = "nul";
  56 #endif
  57
  58     if (!null_fd) {
  59         null_fd = open(device, O_RDWR);
  60         if (null_fd < 0) {
  61             int error = errno;
  62             VLOG_ERR("could not open %s: %s", device, ovs_strerror(error));
  63             null_fd = -error;
  64         }
  65     }
  66
  67     return null_fd;
  68 }
  69
  70 /* Close standard file descriptors (except any that the client has requested we
  71  * leave open by calling daemon_save_fd()).  If we're started from e.g. an SSH
  72  * session, then this keeps us from holding that session open artificially. */
  73 void
  74 close_standard_fds(void)
  75 {
  76     int null_fd = get_null_fd();
  77     if (null_fd >= 0) {
  78         int fd;
  79
  80         for (fd = 0; fd < 3; fd++) {
  81             if (!save_fds[fd]) {
  82                 dup2(null_fd, fd);
  83             }
  84         }
  85     }
  86
  87     /* Disable logging to stderr to avoid wasting CPU time. */
  88     vlog_set_levels(NULL, VLF_CONSOLE, VLL_OFF);
  89 }